This invention relates to process control. In one aspect this invention relates to a method and apparatus for maintaining a desired unreacted monomer concentration in a polymerization reactor.
In a typical polymerization reaction, monomer, diluent and catalyst are fed to a reactor where the monomer is polymerized. The diluent does not react but is typically utilized to control solids concentration and also to provide a convenient mechanism for introducing the catalyst into the reactor. The reactor effluent, a mixture of polymer, diluent and unreacted monomer, is removed from the reactor and fed to a flash tank where the polymer is separated from the diluent and unreacted monomer. Typically, catalyst will be contained in the polymer.
It is important to control the concentration of the unreacted monomer within the reactor because some of the properties of the polymer produced, such as melt index, are highly dependent on the unreacted monomer concentration. Typically, the unreacted monomer concentration has been controlled by manipulating the monomer feed rate, the diluent feed rate, or the catalyst feed rate.
Additionally, for economical operation it is desirable that the unreacted monomer in the reactor effluent be recovered and recycled to the polymerization reactor. Processes for the recovery and recycling of the diluent have been disclosed in U.S. Pat. Nos. 3,639,374 and 4,424,341. Such processes are generally one-stage flash processes with a low pressure recovery of the monomer and diluent after the flashing, a one-stage flash process with a high pressure recovery of the diluent and the monomer within the reactor after the flashing, or a two-stage flash process with the recovery of a major portion of the monomer and diluent after the high pressure flash while the diluent and monomer are still at a sufficient pressure to avoid needing recompression.
In a one-stage flash process with a low pressure recovery, a slurry of polymer and diluent is collected in one or more settling legs of the polymerization reactor. Next the slurry is discharged to a flash vessel wherein the mixture is flashed to a low pressure such as about 20 psia. While the flashing results in substantially complete removal of the diluent and monomer from the polymer, it is necessary to recompress the vaporized diluent and monomer in order to be able to condense them to liquid suitable for recycling to the polymerization reactor. The cost of recompression and the utilities for its operation often amounts to a significant portion of the expense involved in producing the polymer.
When a high pressure flash is utilized, recovery of the diluent and monomer after the high pressure flash reduces the need for recompression of the vaporized diluent and monomer before recycling to the polymerization reactor. Thus, the use of a high pressure flash results in a reduction of the expense involved in producing the polymer. In a one-stage flash with low pressure recovery, direct sampling of the monomer and diluent gas gives accurate analysis of the amount of monomer in the polymerization reactor. However, when a high pressure flash recovery is utilized, a significant portion of the monomer is retained with the polymer solids so that a direct sampling of the gases from the high pressure flash tank will result in an inaccurate analysis of the amount of monomer in the polymerization reactor. Thus, it would be desirable to develop an accurate system of determining the amount of monomer in the polymerization reactor for a polymerization system using a high pressure flash, in order to control the concentration of the unreacted monomer in the polymerization reactor and, also, realize the cost benefits associated with utilizing a high pressure flash.